Factors affecting variable weld penetration

Abstract

The published literature on variable weld penetration has been reviewed and evaluated, with particular respect to autogenous TIG and laser welding processes. It was concluded that the Heiple-Roper theory provides a satisfactory explanation for variable weld penetration, namely, that the direction and the strength of the fluid flows in the weld pool caused by Marangoni convection are responsible for the large differences in weld penetrption observed with different casts. The strength and direction of these flows are determined by the sign and the magnitude of the temperature coefficient of the surface tension dγ/d T. The sign of dγ/d T is determined by the concentration of specific surfactants (S,O, and Se), in the metal, e.g. dy/d T is negative when S < 50 ppm and positive when S > 70 ppm. However, reactive elements such as AI, Ca, and Ce can have an adverse effect on penetration by reducing the soluble sulphur (or O or Se) content. Various observations of the effect of steel composition on weld penetration have been explained on thermodynamic grounds. The direction of the fluid flow in the weld pool is also responsible for other welding problems, namely, arc wander, porosity, and humping, etc. In addition to the Marangoni forces there are three other flow mechanisms operating in the weld pool, namely, electromagnetic (Lorentz), buoyancy, and aerodynamic drag forces. Mathematical models, which have been developed to describe the effects of these forces on the overall fluid flow and the temperature profiles in the pool, are briefly described. All of these models predict that Marangoni convection is the dominant flow mechanism under normal welding conditions. Changes in welding procedures (e.g. current, welding speed, arc length, etc.) have been analysed in terms of their effects on the four fluid flow mechanisms and it is necessary to consider high and low sulphur casts separately when attempting to improve penetration by changes in welding procedure. Recommendations are made for changes in welding procedure to provide (a) improved weld penetration and (b) more consistent weld profiles when using casts with different sulphur levels.